Photosynthesis is a process whereby light energy is converted into chemical energy as carbohydrates in plants. The process begins by light energy capture in a chlorophyll-containing protein complex called Photosystem II (PSII), followed by electron transfer along a chain of carriers; the electron-deficient hole left behind has its electron replenished via a tyrosine residue and finally by a Mn4CaO5 cluster, which oxidises water into oxygen molecules, electrons and protons. The electron and proton equivalents are then transferred to other parts of the photosynthetic machinery to drive the fixation of CO2 into carbohydrates.

In the lab, a goal of artificial photosynthesis is to be able to mimic the function of PSII. To this end, you need a pigment that is able to absorb light and perform electron transfer or charge separation. Then you need to be able to pass that charge deficit along to a catalyst, which will oxidise water into oxygen, protons and electrons. Finally, you need a separate catalyst to convert the protons and electrons into a useful form, for instance hydrogen.

How will it be useful to the human race?

If we can mimic the functionality of PSII, we can harvest energy from sunlight and convert it to useful forms of energy, such as electricity. The Sun is the most abundant and clean energy source that is freely available; the power supplied from the Sun to the Earth is about 10,000 times the rate of current energy consumption by the world population.

Could it be used as a source of renewable energy?

Yes. It is a renewable energy source. Sunlight is harvested to oxidise water into protons and electrons, which can be combined to form hydrogen. We can use hydrogen as fuel in a combustion chamber or a fuel cell to generate electricity and the by-product is water. So we have an elegant cycle where the fuel is made from water and the by-product from using this fuel is also water.

What stage is it at, at the moment?

Many scientists are working on the problem but we are still a long way from a practical solution. In our lab1 we have recently demonstrated charge separation and transfer by engineering a ferritin molecule in which the haem is replaced by a pigment molecule, Zn Chlorin, and the iron is replaced by manganese atoms to mimic PSII. When exposed to light we see there is charge transfer from Zn Chlorin to one of the tyrosine residues present in the ferritin molecule. This represents the first key step in mimicking PSII function, charge separation and transfer, in a bio-engineered system. The next step is to connect this ferritin molecule to a suitable water oxidation catalyst. Others have taken a fully synthetic approach2 using semiconductors and metal oxides as photocatalysts while some have opted for a hybrid approach3 where PSII and synthetic photocatalysts are combined to produce hydrogen.

Dr Michael (M.H.) Cheah and Prof. Fred (W.S.) Chow
Division of Plant Science, Research School of Biology,
College of Medicine, Biology & Environment,
The Australian National University